Is it possible to render a file with combined CPUs of 4 systems?
If it's a long render like an interior visualization with lots of indirect light and it takes hours to render it pays off to just split the samples in 4 parts, so if you need 20 000 samples you can just render 5000 on each machine with different render seeds saving them in EXR 32bit full float format. Then you need to combine everything mixing them proportionally - so you would blend each 2 of them with weight of 0.5 and then blend the results together with 0.5 weight as well in Blender's compositing so each of them contribute 1/4th to the result. Obviously if you are rendering animations that will be too much work so you would need to look into automating this with Python, but with animations you can just render different frames on different machines as well. There might be addons for automating this already that I am not aware of, so it is probably a good idea to search for them as well if manually doing this is too much trouble for you.